The study of the mechanism of regulation of DNA replication and its coordination wit cell division is of obvious importance proliferation such as cancer. Am important, but incompletely understood step of this larger problem is the mechanism and control of replication terminating, and its possible link to cell division. Utilizing E. coli and B. subtilis and the drug resistance plasmid R6K as model systems, we propose to study the mechanism of action and regulation of the terminator proteins. Specifically, we propose to investigate the molecular basis of the polarity of the contrahelicase activities of ter proteins by combined genetic and biochemical approaches. We have recently discovered that the activity of the ter protein is modulated by specific protein-protein interaction with a novel antiterminator protein. We propose to clone, overproduce and purify to homogeneity the anti-ter protein, and to study the biochemistry and physiological consequences of its interaction with the ter protein. Finally, we have crystallized the ter beta protein (RTP) of B. subtilis, and the crystals diffract to a resolution of 2.5A. We propose to solve not only the structure of the apoprotein but also that of the protein-DNA complex.